A phased array includes an array of ultrasound transducers, each of which produces ultrasound in response to receiving a control signal from radio frequency (RF) driving circuitry. The RF driving circuitry electronically adjusts the phase, and amplitude of the ultrasound produced by the transducers so that the phased array can dynamically focus the ultrasound without being moved mechanically. To enable the array to focus the ultrasound in three dimensions, the transducer size should be sufficiently small to provide an ultrasound field that covers the entire volume in which the focusing is desired. Decreasing the size of the transducers, however, increases their electrical impedance, which in turn causes mismatch between the RF driving circuitry and the transducers. Conventional methods for reducing impedance mismatch include adding a matching circuit between each transducer and the RF driving circuitry. As the number of elements within an array increases, supplying a matching circuit for each element becomes costly.
Other conventional techniques for improving electrical-impedance matching include assembling the transducer from multiple layers having a combined electrical impedance that matches the impedance of the RF driving circuitry. Manufacturing such transducers can be complicated and costly, and their usefulness for therapeutic applications has not been demonstrated.